In recent years, in accordance with an improvement of a performance of a computer, it has been possible that a plurality of virtual computers (hereafter referred to as a virtual machine) can be operated on a physical computer. The virtual machine activates an operating system that has been installed to a virtual storage device (hereafter referred to as a virtual hard disk drive) and executes a wide variety of processing.
On the other hand, a technique for acquiring a snapshot that can be written on a file system is publicly known (see U.S. Pat. No. 6,857,011). The snapshot means a static image of data at a certain point of time.
In accordance with the technique of a snapshot that can be written, the difference data is generated by a write to a snapshot. The difference data is stored into a storage region other than a storage region in which a snapshot has been created. A stored destination of the difference data is changed to the other storage region.